The present invention relates to a turret for a vessel such as a drilling or production vessel for recovery of oil offshore, such turret being erected so as to allow rotation in a throughgoing opening or well in the hull of the vessel, and having suspension arms which are equipped with axially and radially provided bearing elements which operate in relation to corresponding bearing elements on the vessel.
A turret of the abovementioned type is normally fitted with bearing elements with spring devices to assure an even distribution of the bearing forces. The suspension arrangements of the bearings have a fairly large slack, partly to absorb elongation in the vessel, and are often jointed to handle angular deformation and to even out loads. In order to achieve the best possible control of suspension forces and deformation in the bearings, the vessel and rotary or tower turret, complicated mechanical or hydraulic solutions are often used. A hydraulic solution is shown in EP Patent Application No. 0,207,915. It consists of an upper radial bearing, an axial bearing and a lower radial bearing. Each of these bearings consists of a large number of hydraulic piston/cylinder devices which are each mounted on a bearing element.
One major disadvantage with these solutions is that they are complicated, and therefore expensive to build and maintain. A further disadvantage is that the bearing surfaces are subject to wear as a result of relative movements and constructional deviations which are due to the suspension/bearing wheel arrangement and to movements in the vessel. With regard to the wheel arrangement, because of large relative movements pommelled on curved surface wheels have to be used. These pommelled wheels have limited bearing capacity, and in the case of large, heavy rotary bearing structures, slide bearings therefore have to be used, or a combination of wheel and slide bearings.
One disadvantage with slide bearings, however, is that large machinery is required to turn the turret, and special, expensive precautions have to be taken to protect the bearings against the corrosive environment on board vessels at sea.
Norwegian Patent No. 165,285 shows a bearing system for a turret in which an attempt is made to eliminate the wear and tear on the radial bearing by using a structural suspension. However, this structural suspension has limited independent suspension, particularly in the case of large, heavy turret, which is necessary to maintain a satisfactory load distribution without using special mechanical or hydraulic springs in connection with the axial bearing elements. The wear and tear on the radial bearing surfaces is thus not quite eliminated by this solution.
Further, with regard to the aforementioned Norwegian patent, mechanical suspension is also used in the radial bearing of the turret, and this suspension is, as previously mentioned, costly to build and maintain, and will cause wear and tear on the axial bearing.